(Adapted from the applicant's abstract) Bone marrow transplant (BMT) is followed by a period of profound and clinically significant immune deficiency. Post-BMT T lymphoid reconstitution recapitulates fetal ontogeny in several ways, including release of thymocytes into the periphery, limited TCR diversity, and cytokine dependent responses to mitogens and antigens. The observed delay in immune reconstitution is a major problem in clinical BMT and will most likely be important for hematopoietic stem cell (HSC) gene therapy directed at immunologic diseases, e.g., SCID or AIDS. The development of hematopoietic growth factors, e.g., GM-CSF, G-CSF, EPO, that stimulate myeloid and erythroid engraftment has resulted in clinical benefit following BMT. Research into cytokines that stimulate lymphoid engraftment has not be under- taken. We have shown that interleukin-7 (IL-7) normalized mitogen and antigen induced proliferation following BMT, and in vivo treatment of mice post-BMT stimulates T lymphoid engraftment and function. To test the effects of IL-7 in vivo, we established a murine model of BMT followed by subcutaneous administration of IL-7. Using this model in a syngeneic BMT setting, we have demonstrated that IL-7 treatment increases the cellularity of the thymus by Day 28 post-BMT to >40X more than control animals; normalizes the thymic subpopulations; induces the proliferation of the immature CD3-CD4-CD8-thymocytes; and induces the early (D28) generation of mitogen-responsive T lymphocytes, without the requirement for exogenous cytokines. These results indicate that IL-7 production post-BMT may be a rate-limiting step in T lymphocyte development and suggest that it can be overcome by exogenous IL-7 administration. The hypothesis addressed by the present proposal is that IL-7 enhances immune reconstitution by inducing the proliferation of an immature population of thymocytes that have not yet undergone thymic selection. We will test: 1) the importance of normal thymic stroma in IL-7-mediated immune reconstitution; 2) the effect of IL-7 on the maturation and diversity of the TCR repertoire post-BMT; 3) the additive of synergistic effects of stem cell factor (SCF) on IL-7-mediated immune reconstitution; 4) whether IL-7 treatment post-BMT increases the susceptibility to graft-versus host disease in a minor histocompatibility antigen system; and 5) whether stromal cells engineered with a retroviral vector to express IL-7 can be used to enhance immune reconstitution. The results from these studies will be important for both HSC transplantation and gene therapy for immunologic diseases.